Over the years, polymers have been widely used in both household and industrial applications. For example, the surface appearance, physical and mechanical properties of polystyrene resins have made them popular in a variety of fields including electronics, automobiles, as well as sundry and sanitary items. Similarly, polyolefins such as polyethylene and polypropylene, have been widely used in extrusion and injection molding applications.
The art has long sought to introduce additives, both liquid and solid, into polymers. To this end, masterbatches, e.g., solid dyes and pigments in a polymer matrix, have been used to introduce solid additives, e.g., a desired color, into a polymer.
Alternatively, liquid active ingredients have been introduced into the polymer compositions in order to provide or enhance a desired property of polymer compositions. Examples of "active" liquid agents include bactericides, perfumes, insectifuges, rust preventatives, mildew-proofing agents, and antimicrobials, among others.
One particularly significant problem faced in producing extruded or molded articles from resin materials has involved bacterial and microbiological fouling. This problem can be particularly pronounced since resins are often used in high moisture and humidity environments that serve as excellent breeding grounds for unwanted microorganisms such as bacteria, mold, fungi and mildew.
In an attempt to address this problem, a variety of processes have been developed each of which seek to introduce an antimicrobial or biocide agent into a molded resin article. In this regard, it has been proposed to mix the desired agent into the synthetic resins material prior to formation of the molded articles. Alternatively, processes have involved introducing the agent during or subsequent to formation of the articles. Examples of such process include applying the desired active agents onto the surface of the synthetic resin product, injecting the agent into a "space" within a formed resin product or laminating the agent between layers or sheets of the resin(s).
Specific techniques for introducing antimicrobial agents and/or biocides into polymer resins have been disclosed in U.S. Pat. No. 3,531,433, to Elmer, U.S. Pat. No. 3,987,007, to Kalogris, U.S. Pat. No. 4,725,657 to Shibanai, U.S. Pat. No. 4,938,955 to Niira et al., U.S. Pat. No. 5,205,016 to Ohmae et al., U.S. Pat. No. 5,358,979 to van Hoboken et al., U.S. Pat. No. 5, 482,989 to Koskiniemi, U.S. Pat. No. 5,516,814 to Trotoir, and U.S. Pat. No. 5,614,568 to Mawatari et al.
These techniques, however, have suffered from a variety of disadvantages. For example, attempts to introduce the active agents into the resins themselves have been faced with problems such as poor heat stability, toxicity, and a short life. Thus, these techniques have not proven entirely effective in providing a polymer product having the desired antimicrobial effect.
Moreover, these techniques are often severely limited in terms of the types and amounts of antimicrobial agents that may be introduced into the resin. For example, conventional processes using extruders to mix a polymer with an additive are significantly limited in terms of the polymers and additives that can be used. That is, where melting point for a resin is greater than the boiling point for a liquid additive, the components are said to be incompatible because the temperatures needed in the extruder would prevent the additive from becoming mixed with the resin.
One solution to this problem has involved the modification of the extruder so as to introduce certain additives immediately preceding the point where the polymer exits the extruder. While this arrangement has only worked for certain lower boiling point additives, it is also extremely limited in terms of the amounts of additive that may be introduced into the polymer. Moreover, it requires the use of very specialized equipment that increases production costs.
Even in those situations where the polymer and additive may be "compatible", the amount of additive is typically limited to 5 weight percent or less, although as much as 10 weight percent has been suggested under certain limited conditions.
Recent attempts to increase the amount of the liquid additive have focused on the use of carriers, e.g., plasticizers such as those disclosed in U.S. Pat. No. 5,358,979, for the active agent, in this case, microbiocides. However, such carriers have introduced their own set of problems regarding the use and handling of the resulting masterbatch.
Accordingly, the need still exists for a method which is capable of providing increased amounts of liquid active component in a polymer without the need for carrier materials.